On the Nature of the Structural and Magnetic Phase Transitions in the Layered Perovskite-Like (CH₃NH₃)₂[Fe^IICl₄]

In view of renewed interest in multiferroic for molecular systems, we re-examine the structural and magnetic properties of the potentially ferroic layered perovskite-like (CH₃NH₃)₂[Fe^IICl₄] due to its high-temperature magnetic ordering transition. The structures from several sets of diffraction data of single crystals consist of square-grid layers of corner-sharing FeCl₆ octahedra and changes from the high-symmetry <i>I</i>4/<i>mmm</i> (<i>T</i> > 335 K) to the low-symmetry <i>Pccn</i> (<i>T</i> < 335 K). In the former the iron and bridging chlorine atoms are within the layer and the organic cations sit in the middle of each square grid, while in the latter the octahedra are tilted in pairs, two in and two out, progressively by up to 12° and the nitrogen atoms follow their motion to be nearer to the two-in pairs. Crystals are stable up to 450 K and display three phase transitions, two structural at 332 and 233 K and one magnetic at 95 K. The temperature dependences of the dc magnetization (zero-field and field-cooling modes) in different applied fields (10–10 000 Oe) on several aligned single crystals independently reveal a hidden-canted antiferromagnetic ground state of at least four sublattices and not the reported canted antiferromagnetic ground state. A metamagnetic critical field of only 200 Oe transforms it to a canted antiferromagnet. The estimated canting angle is 1.4° in zero field, and it folds to ca. 2.8° in a field of 50 kOe at 2 K. The easy axis is along 010, the hard axis is along 100, and the intermediate and canting axis is 001. Using the available extracted parameters the phase diagram has been constructed. This study provides evidence of a complex and intricate manifestation of the orientation, temperature, and field dependence of the interplay between anisotropy and coherent lengths, which would need further studies